The Greek man, Dimosthenis Katsigiannis Karkasi, was accused of operating from his pizzeria near the hospital to find potential organ "donors" with financial problems and linking them up with the doctors in exchange for money.

The alleged ring was busted in 2013 with the arrests of the five men, who were suspected of having illegally carried out 14 kidney transplants.

Prosecutors were to call on 25 witnesses to substantiate their case -- including a former policewoman who was said to have worked with the ring after selling her own kidney, and who accepted to give evidence against the five accused.

If convicted, the doctors and Katsigiannis Karkasi risk prison for the traffic of persons.

For the pizzeria owner, that could be six to 10 years behind bars. For the doctors, because the charges involve abusing their professional status to commit a crime, the sentences could be eight to 16 years.